In pressurized systems, various types of pressure relief valves (also called "safety relief" valves) are used to release pressure so as to avoid damage to the system. These systems are common in oil field operations.
Various types of pressure relief valves are commercially available. Some types use so-called rupture discs which are metal or plastic diaphragms that rupture to allow the gas or fluid to escape. Generally, the pipe or vessel where the rupture disc is located must be depressurized in order to replace the burst diaphragm.
Flapper-type valves are commonly used for large vessels having low relief pressure settings. So-called conventional-type relief valves are also commonly used. These are essentially spring-loaded disc valves. These are commercially available from several manufacturers. "Pilot"-type pressure relief valves are also used. These retain pressure by air pressure which in turn is retained by a very small relief valve.
A still different type of pressure relief valve uses a shear pin to keep the valve closed until an excessive pressure occurs. At that point the pin shears and permits the relief valve to open, thus relieving the pressure. Two of the commercially available shear-in type relief valves use standard U.S. penny-weight nails as shear pins. These valves, therefore, have different settings depending on which penny-weight nail is used, and depending on whether two nails are used instead of just one nail. With such valves, dangerous overpressure of the pressurized system can occur if pins or similar devices of high shear strength should be substituted for nails of low shear strength.